Papr reduction

ABSTRACT

Method of communicating frames of digital data by OFDM modulated signals comprising a first plurality of payload carrying sub-channels and a second plurality of pilot carrying sub-channels, whereby consecutive frames of payload data is are associated with a given pilot configuration and transmitted. Prior to the transmission of a frame of payload data, each of the plurality of pilot configurations are evaluated with regard to PAPR, whereby the pilot configuration being associated with the lowest PAPR value is being chosen for transmission.

FIELD OF THE INVENTION

This invention pertains to the area of wireless radio access technology.More particular, the present invention concerns methods and systemsusing orthogonal frequency division multiplexing (OFDM), such asWireless LANs (WLAN).

BACKGROUND OF THE INVENTION

A well-known problem in OFDM modulation is high PAPR (Peek to AveragePower Ratio) values. High PAPR values occur since it is not possible tocontrol the power level for each symbol when constructing the OFDM(Orthogonal Frequency Division Multiplex) waveform.

The optimal situation for error free transmission is to have a stableconstant PAPR level throughout the transmission of individual packets,in order for the power amplifier (PA) to work well. The PA is typicallylinear only over a limited range in power level, and thus highfluctuations in PAPR level causes the PA to behave non-linearly.Non-linearity of the PA is devastating for the Bit error rate(BER)/Packet error rate (PER) of QAM (Quadrature AmplitudeModulation)-signalling.

There are well known solutions to the classical problem of excessivelyhigh PAPR levels, namely to re-code the data, re-scramble the data or toinsert extra data bits that “levels out” the original data and creates amore favourable PAPR level. However, there are drawbacks to thesesolutions.

Typically, the known solutions are computational intensive, introducedelays, or introduce extra bits that decrease the data rate. There areno known algorithms how to recode or introduce “compensation-bits” andthus trial-and-error must be used.

The current standard for WLAN IEEE802.11 is about to gain success inbeing wide spread to customers with the purpose of replacing wiredEthernet LANs with wireless access. The current deployed standard802.11b, is using the 2.4 GHZ unlicensed band. It is forecasted that ifthe current rate of deployment continues, the spectrum in the 2.4 GHzband will soon be insufficient and that a migration to 5 GHz and 802.11awill take place. The 802.11a specification uses OFDM signalling at thePHY (physical) layer. The OFDM modulated 802.11a PHY layer is sensitiveto fluctuations in PAPR level.

Recently, a IEEE 802.16 Study Group on Mobile Broadband Wireless Access(MBWA) IEEE 802.16, has addressed radio access for stations in fastmoving vehicles with speeds up to 200 mph. However, it is not possibleto use the 802.11a physical (PHY) layer for mobile stations moving at ahigh speed and being exposed to adverse signalling conditions.

The MBWA requires pilot symbols evenly distributed—at a certainpilot-to-data ratio, PDR—throughout a packet in order for the frequencytracking and channel estimation mechanism to function for fast movingstations.

SUMMARY OF THE INVENTION

It is a first object of the invention to set forth a method fordecreasing the level of fluctuation of the PAPR level but withoutcausing a data rate penalty.

This object has been achieved by the subject matter set forth by claim1.

It is a further object to set forth a method of transmission that caneasily be implemented in a transmitter, the method providing very lowdelay.

This object has been accomplished by the subject matter of claim 2.

It is a further object to set forth a method, which allows for a low PDRand thus a high throughput, but still providing a robust detection.

This object has been accomplished by the subject matter of claim 3.

It is a further object to set forth a transmitter providing robust datatransmission and low PAPR values.

This object has been achieved by the subject matter of claim 9.

It is a further object to set forth a transmitter, which moreover iscost effective and provides low delay.

This object has been achieved by the subject matter of claim 10.

It is a further object to set forth a receiver allowing PAPR efficienttransmission to be accomplished.

This object has been achieved by the subject matter of claim 12.

Further advantages will appear from the following detailed descriptionof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a typical transmitter,

FIG. 2 shows a typical receiver,

FIG. 3 shows an OFDM modulation scheme,

FIG. 4 discloses a transmitter according to a first embodiment of theinvention,

FIG. 5 discloses a transmitter according to a second embodiment of theinvention,

FIG. 6 discloses a receiver according to the first and second embodimentof the invention,

FIG. 7 shows pilot configurations according to a first embodiment of theinvention,

FIG. 8 is a schematic illustration of the PAPR level for two alternativepilot configurations,

FIG. 9 shows a format for transmitted/received frames according to afirst embodiment of the invention,

FIG. 10 relates to the calculation of pilot configurations according tothe first embodiment of the invention,

FIG. 11 shows pilot configurations according to a second embodiment ofthe invention, and

FIG. 12 shows pilot configurations according to a further embodiment ofthe invention

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

In FIG. 1, a conventional transmitter has been shown, comprising abuffer stage 1 for intermediately storing incoming payload data arrangedin frames to be transmitted. The transmitter comprises a mapping stage(2) in which the payload data is mapped into complex symbols beingdefined by real, I, and complex, Q, components using for instance a BPSKor QAM modulation scheme. The transmitter moreover comprises a pilotinsertion stage for interspersing appropriate pilot symbols in thestream of symbols carrying the payload data; an Inverse Fast FourierTransform stage (IFFT) for mapping the BPSK or QAM modulated symbolsonto respective sub-carriers and subsequently transforming the frequencydomain signals to a time domain signal; a cyclic prefix (CP) insertionstage (5) providing a cyclic addition of the signal for facilitatingsuppression of multipath effects in the receiver stage; a basebandfilter 6 for out-of-band suppression; a digital to analogue converter(DAC) (7); and a radio frequency transmit stage (RF TX) (8) forup-converting and amplifying the baseband signal to a high frequencytransmit signal.

In FIG. 2, a conventional receiver has been shown comprising a receiveamplification input stage RF RX 12 an analogue to digital conversionstage (ADC) 13 for converting the high frequency signal to a basebandsignal; a baseband filter stage 14; a cyclic prefix removal stage 15; afrequency offset compensator 16 a Fast Fourier Transform stage 17 fortransforming the baseband signal to frequency domain signals relating tothe individual sub-channels. The receiver moreover comprises ademodulation stage 18 for decoding the information (BPSKI QAM) of theindividual frequency domain signals back into bit estimates; using achannel estimation signal output from a channel estimator stage 21.Finally, a decoding stage 19 decodes the received signal into theoriginal data frame format

In FIG. 3, the frequency domain signal being output from the inversefast Fourier transmission stage 4 of the conventional transmitter ofFIG. 1 has been shown. As shown, the spectrum is divided into aplurality of orthogonal carrier channels, comprising a number of payloaddata channels PL and four pilot signals P1-P4. Similarly to the IEEE802.11a standard, there may be 48 payload carrier channels and 4 pilotchannels whereby the payload signals may be Quadrature AmplitudeModulated (QAM or any higher order of QAM modulation (n-QAM)) while thepilot signals may be binary phase shift keyed (BPSK).

First Embodiment of Transmitter

In FIG. 4, a block diagram of the transmitter according to a firstpreferred embodiment of the invention has been shown. It should beunderstood that stages 6, 7 and 8 are the same as those shown withregard to FIG. 1. According to the invention—periods of data frames areintermittently split by splitter 21 into identical units 25 and 26.

Having regard to unit 25, the data is stored in buffer 10, wherefromdata is read out in a predetermined order into mapper 20 from which datais subsequently being provided in parallel to a plurality a pilotinsertion stages 3_1, 3_2 . . . 3 _(—) n.

In each respective pilot insertion stage, a predetermined configurationof pilot signals, PC, is applied to the various predeterminedsub-carriers P1-P4. The configurations in each stage shall be differentfrom one another. The individual pilot symbols, on the other hand, maybe chosen arbitrarily as long as the configurations are unique. Thepayload data channels may be the same as shown in FIG. 3.

In FIG. 7, four exemplary BPSK pilot configurations being designated 00,01, 10 and 11 have been shown for pilot channels P1-P4. Each pilotconfiguration is being inserted in a respective pilot insertion stage,such that stage 3_1 makes use of pilot configuration 00; stage 3_2 makesuse of pilot configuration 01 and so forth. For instance in theconfiguration 00, the first pilot P1=−1, the second pilot P2=1; P3=1 andP4=−1.

As appears, each respective signal from the pilot insertion stage 3_1-3_(—) n is subsequently processed in respective control word insertionstages 5_1 to 5 _(—) n, whose function shall be described later and toIFFT stages 4_1-4 _(—) n such that respective frequency domain signalsare provided.

The insertion of a given pilot configuration in the stream of payloaddata will give rise to a specific output signal from the respectiveInverse Fast Fourier Transmission stage.

In FIG. 8, a first schematic output signal C1 corresponding e.g. to theconfiguration 01 being associated with a first PAPR value PAPR_1 and asecond schematic output signal C2 corresponding e.g. to theconfiguration 10 being associated with a second PAPR value PAPR_2 havebeen illustrated.

As appears, the PAPR values differ because of the variations in thepilot configurations.

According to the present invention, the PAPR evaluation and pilotdecision stage 13 carries out an evaluation of the PAPR values asprovided by the respective IFFT stages 4_1-4 _(—) n and chooses thepilot configuration, which is associated with the lowest value andstores results temporarily.

The idea here is that from a PAPR point of view it is advantageous tosubstitute fewer 20 sub-carriers more often in order to obtain a givenpilot-to-data ratio (PDR). If there is more than one pilot configurationthat can be chosen, it is possible to choose the configuration thatminimises the PAPR. From the receivers point-of-view it is not importantwhich pilot configuration is chosen, only that it is known.

Parallel to the pilot insertion stages 3_1-3-n, the delay stage 9 storesa predetermined number of payload data frames, each frame comprising thepayload data, which is to be transferred over the payload channels.

Internally in the PAPR measurement and pilot decision device 13, thedata frames comprising the id+ portion are intermediately stored. Whenall frames are processed as indicated at time t5 in FIG. 10, the chosenpilots and control words are inserted in frames to be transmitted incorrect order in step 10 and step 11, respectively.

There are two methods to let the receiver know the specific bit patternof the pilot configuration: Either specific information of which pilotconfiguration is to be used is signalled in advance by the transmitteror such information is derived directly from the pilot configuration bythe receiver.

According to the first embodiment of the invention, information aboutthe chosen pilot configuration is transmitted in advance. Thisinformation denoted control data is inserted on one predeterminedpayload channel, PL, in stage 11 and stages 5_1-5 _(—) n of FIG. 4.

In FIG. 9, the frame format of received data has been shown for incomingframes B_(n-1) to B_(n+p+1). One part of the frame Id+, as coded on oneof the data payload carriers, is a control word that indicates whichpilot configuration, PC, will be used in a subsequent frame or in aframe of any predetermined subsequent order number.

By way of illustration, the other portion of the frame, PL, containdata, which has been coded according to a respective pilotconfiguration.

In FIG. 10, an exemplary illustration of a given stream of data frameshas been shown. Stages 9, 10, 11 and 13 are performing calculationsaccording to a certain frame period FP. In FIG. 10, the frame period is,by way of example, set to six frames. The processing of PAPR detectionand evaluation in stage 13 is initially performed on frame Bn+5 (orBn+p) at time t1. However, before the PAPR evaluation is carried out, apredetermined default pilot configuration is inserted as control word(id+). In the present example, the pilot configuration designated by 01is inserted. Hence, a given pilot configuration is estimated for frameBn+5. This information is coded as control word on the preceding frameBn+4 via control word insertion stages 5_1-5 _(—) n and 11. Since nowone of the 25 payload channels are coded with a control word id+, theremaining payload data of frame Bn+4 must be PAPR evaluated with respectto the inserted control word. At time t2 this evaluation is carried out,and by way of example pilot configuration 11 is found to optimise PAPRvalue.

Hence, the transmitter processes the buffered frames in opposite orderto the incoming frames. When frame Bn is reached at time t5, a defaultpilot configuration, dft, is used, such that synchronicity can beobtained for a subsequent frame period, FP.

It appears that every 6'th frame word will not be optimised with regardto PAPR. Hence, 35 a compromise will have to be made between processingdelay and PAPR value as determined by the frame period.

Returning now to FIG. 4, the optimised pilot signal is inserted on agiven frame B in stage 10, while in stage 11, the control word id+ asgiven by the evaluation unit 9 is inserted on one of the payloadchannels PL enabling subsequent detection.

As mentioned above, the other unit 26 carries out the same processes asdescribed above on a subsequent period of frame and in this manner,units 25 and 26 work intermittently and secure that PAPR optimisedframes are being transmitted. It is noted that the transmission involvesa certain delay mainly depending on the frame period FP.

It is noted that many variations exist as to the number of frames beingbuffered or to which particular frame of a subsequent given order numberthe control word is associated.

First Embodiment of Receiver

In FIG. 6, first and second embodiments of the receiver according to theinvention have been shown. According to the first embodiment of thereceiver according to the invention, which is meant for operationtogether with the transmitter according to the first embodiment, acontrol word extraction stage 23 is provided (According to a secondembodiment of the receiver, the above control word extraction stage 24is replaced with a pilot extraction stage 23).

The control word extraction stage 24 extracts the control word, id+,from the output of the demodulator 18. The pilot reference generator 25transforms the pilot configuration information into the correspondingBPSK symbols for each pilot sub-carrier, e.g. according to the controlinformation as set out in the table of FIG. 7.

From signals generated by the pilot reference generator above, arespective frequency reference signal, which is necessary for thefrequency estimation stage 17 and a respective channel reference signalfor the channel estimation stage 21, are provided, such that correctdecoding can be performed at stage 19.

Second Embodiment of Transmitter

According to a further embodiment of the invention, the pilotconfigurations are formed as block codes, that is, codes, which even ifexposed to a certain amount of changes to individual bits in the blockor pilot configuration, will allow for correct interpretation. In FIG.5, a transmitter for the second embodiment has been shown.

If the sub-set of allowed pilots to the set of possible pilots issufficiently small, i.e. the Hamming distance between the pilots arelarge enough, the receiver can determine which pilot was transmittedeven in the case that some bit errors should occur in the pilotconfiguration. In this way, there is no need to signal in advance whichpilot symbol that is used.

As above, the transmitter calculates the PAPR for each of the pilotconfigurations and transmits the best one. If it is specified that onlyspecific block codes are allowed for transmission, it is possible todetermine in the receiver which code was transmitted even in thepresence of errors. If more pilot sub-carriers are used per OFDM symbol,it is possible to use longer codes with better error correctingabilities.

FIG. 11 relates to the transmitter of FIG. 5 wherein only two pilotconfigurations are used, and hence only one out of two PAPR values canbe chosen for optimisation. The code of FIG. 11 is a simple repetitioncode, which has an error correcting capability of 2.

The number of pilots can also be increased such that the error codingcapability increases and the PAPR minimisation capability increases,although at the cost of an increased overhead. FIG. 12 is such anexample in which the hamming distance of codes is larger or equal to 3.

It is noted that the transmitter according to the above embodimentcorresponds largely to the transmitter shown in FIG. 4 except that thecontrol word insertion stage 11 and the other means for various bufferoperations (the processing shown in FIG. 10) are not provided.

Instead, incoming data is processed directly and fed in parallel to thepilot insertion stages 3_1-3 _(—) n, such that the examination of PAPRvalues as explained with regard to the first embodiment of thetransmitter can be carried out.

The delay stage 9 stores a predetermined number of payload data frames,each frame comprising the payload data that is to be transferred overthe payload channels.

The delay is timed such that the choice of pilot configuration asindicated by PAPR measurement unit and pilot insertion unit 13 can beinserted on the actual frame on which the evaluation was performed.Hence, in stage 10 the chosen pilots are inserted.

It is noted that the transmitter is of a simpler construction than thefirst embodiment. Moreover, the delay in the transmitter has beenconsiderably reduced.

Compared to the first embodiment of the transmitter, this method has avery low implementation cost. It is only necessary to process an extraIFFT for each code that is tested. For other methods, e.g. methods usingre-scrambling, more processing is required. Moreover, all processing isperformed on one OFDM symbol or frame at a time. There is no dependencybetween the OFDM symbols or frames and hence no need forsynchronisation.

Receiver—2'nd Embodiment

In FIG. 6, a second embodiment of the receiver according to theinvention has been shown. Apart from the elements described above, thereceiver comprises a pilot extraction stage 23. The pilot extractionstage extracts the assumed pilot configuration from the output of thedemodulator 18. Furthermore, the pilot extraction stage 23 performserror correction of the received pilot configuration. The errorcorrected pilot configuration is forwarded to the pilot referencegenerator 25 for transformation into BPSK symbols.

CONCLUSION

In conclusion, it is noted that the present invention improves PAPRperformance in for instance OFDM modulated transmission systems. Thepresent invention can be readily used, in order to allow datatransmission for fast moving vehicles. The method proposed here showshow to choose the pilots in order to minimise the PAPR with no extrapenalty, such as decrease in data rate. Having regard to wireless localarea networks such as IEEE 802.11a the invention is apt as amodification to the physical layer (PHY) of such existing wireless LANprotocols. The present invention requires no modifications to the MAC(Media Access Control layer) signalling of such protocols.

It should be noted that the present invention is not limited forapplication to wireless LAN systems, but is applicable to systems inwhich a robust data transmission is desirable; hence, systems of whichscope are defined in the appended claims.

1-13. (canceled)
 14. A method of communicating consecutive frames ofdigital data, said method comprising the steps of: mapping payload datainto complex symbols; interspersing appropriate pilot symbols; and,mapping symbols on respective sub-channels; whereby the insertion of agiven pilot configuration into the stream of payload data will give riseto a specific output signal being associated with a given PAPR value;wherein the digital data comprises OFDM modulated signals comprising afirst plurality of payload carrying sub-channels and a second pluralityof pilot carrying sub channels; wherein each individual frame of payloaddata to be transmitted over the payload channels is associated with agiven unique pilot configuration chosen from a sub-set of predeterminedpilot configurations, each pilot configuration forming a unique patternof predetermined pilot symbols and transmitted; wherein, prior to thetransmission of at least one given frame of payload data, each pilotconfiguration of the sub-set is evaluated with regard to PAPR for theassociated frame of payload data, whereby the pilot configuration beingassociated with the lowest PAPR value is chosen for transmission. 15.The method according to claim 14, wherein the plurality of pilotconfigurations represent block codes allowing error correction at thereceiver.
 16. The method according to claim 14, wherein a control wordindicative of the pilot configuration associated with a subsequent frameor a particular frame of a subsequent given order number is insertedinto the frame and coded on a predetermined payload channel.
 17. Themethod according to claim 16, wherein for every n-1 frame in a frameperiod, the complete frame comprising both payload data and the controlword and pilot configuration is optimized with regard to PAPR.
 18. Themethod according to claim 17, wherein every n frame in a frame period isnot optimized with regard to PAPR.
 19. The method according to claim 14,wherein, the sub-carriers carrying the pilot signals are digitallymodulated at a lower order (BPSK) than sub-carriers carrying the payloaddata (QAM).
 20. The method according to claim 15, wherein the block codeforming pilot configurations have a hamming distance of ≧3.
 21. Themethod according to claim 14, wherein the sub-channels are modulated byBPSK or n-QAM modulation.
 22. A transmitter comprising: a mapping stage,mapping payload data on a subset of a plurality of frequency orthogonalsub-carriers; a plurality of parallel-coupled pilot insertion stagescoupled to the mapping stage, each pilot insertion stage inserting aunique pilot configuration on at least another subset of sub-carriers; arespective inverse fast Fourier transmission stage processing signalsfrom each respective pilot insertion stage; a PAPR measuring and pilotdecision stage, measuring and evaluating PAPR for each unique pilotconfiguration; wherein, each individual frame of payload data to betransmitted over the payload channels is associated with a given uniquepilot configuration chosen from a sub-set of predetermined pilotconfigurations, each pilot configuration forming a unique pattern ofpredetermined pilot symbols, and transmitted; and, wherein, prior to thetransmission of at least one given frame of payload data, each pilotconfiguration of the sub-set is evaluated with regard to PAPR for theassociated frame of payload data, whereby the pilot configurationassociated with the lowest PAPR value is chosen for transmission. 23.The transmitter according to claim 22, wherein each unique pilotconfiguration has a hamming distance of at least three to any otherpilot configuration.
 24. The transmitter according to claim 22, furthercomprising a control word insertion stage for inserting a control wordin a transmitted frame, the control word being indicative of the pilotconfiguration used in a frame of any given subsequent order number. 25.A receiver comprising: a fast Fourier transform stage for transformingbaseband signals into frequency signals relating to individualsub-channels; and, a demodulation stage for performing individualdemodulation, such as n-QAM, of the frequency signals into bitestimates; wherein the receiver further comprises a pilot extractionstage for extracting block coded pilot signals into assumed pilotconfigurations; wherein the assumed pilot configuration is provided to afrequency estimator for adjusting the fast Fourier transform stage andto a channel estimator for adjusting the demodulating stage.
 26. Areceiver comprising: a fast Fourier transform stage for transformingbaseband signals into to frequency signals relating to individualsub-channels; and, a demodulation stage for performing individualdemodulation, such as n-QAM, of the frequency signals into bitestimates; wherein the receiver further comprises a control wordextraction stage for extracting a control word of any subsequent orderinto an assumed pilot configuration; and, wherein the assumed pilotconfiguration is provided to a frequency estimator for adjusting thefast Fourier transform stage and to a channel estimator for adjustingthe demodulating stage.